Presently there is great interest in the fabrication of functional semiconductor devices with length scales below 100 nm, i.e., nanoelectronics. The laws of physics allow, in theory, the building of logic devices such as transistors with characteristic length scales on the order of about 1 nm. Reaching these limits, however, is difficult and expensive.
Methods for fabricating devices below 100 nm include both top down and bottom up approaches. Conventional top down approaches such as photolithography or electron beam lithography utilize formation and selective removal of various levels to form functional devices. Top down processes are very expensive for devices with features below 100 nm. In addition, such methods can generally be used only to build two-dimensional logic, typically on a planar silicon wafer, and historically have followed Moore's law yielding only a factor of two increase in device density every 18 months.
Direct-write top down approaches have included atomic force microscopy (AFM) direct writing of liquids, i.e., dip pen nanolithography and scanning tunneling microscopy (STM) writing of oxides and charge replicas. These methods suffer from slow speeds, lack of a general set of building materials for fabricating electronic components, and a constraint to two-dimensional structures.
Tools for creating three-dimensional structures employ, for example, electron beam and ion beam decomposition of chemical vapor precursors. Such tools have been useful in mask and chip repair and have been shown to be capable of writing three-dimensional structures. Typically, organometallic precursor gases adsorbed onto substrate surfaces are decomposed using energy supplied from incident beams, depositing the desired metal or insulator. This technique facilitates deposition of nanometer- to micrometer-size structures with nanometer precision in three dimensions and without supplementary process steps such as lift-off or etching procedures. Although successful in creating high resolution three-dimensional structures, both scanning electron microscopy (SEM) and focused ion beam (FIB) chemical vapor deposition (CVD) suffer from significant contamination by the organic components of precursor gases. Carbon contamination from typical precursor gases may exceed 50%, thus altering device conductivities to levels unacceptable for many desired applications. Device fabrication by energetic-beam CVD is also constrained by an inherently small number of available precursor gases, thus limiting the variety of materials that can be deposited. Finally, because existing processes are serial and sufficient beam energy must be applied to decompose the precursor, deposition speeds are very slow.
In bottom up approaches, layers are selectively applied to (rather than removed from) a substrate. For example, nano-scaled building blocks synthesized precisely by chemistry or other methods may later be assembled by, e.g., self assembly. Presently the complexity of logic which may be built in this way is extremely limited.
Nature is excellent at predicated assembly of complex molecules such as DNA on a scale similar to that of present-day nanostructures. Nature can make precise molecules with enzymes such as polymerase that typically have extremely low error rates by utilizing feedback and performing error correction. However, direct feedback and error correction are seldom implemented in present fabrication processes, and hence the yield of functional devices is low in comparison to functional molecules formed by biological processes.